T lymphocyte activation and migration are critical for normal immune functions, and signaling by the T lymphocyte antigen receptor-CD3 complex (TCR) and chemokine receptors such as CXCR4 are extensively cross regulated. For example, CXCR4 costimulates the immune activation of TCR-stimulated T lymphocytes, while TCR signaling abrogates CXCR4-mediated migration and participates in CXCR4-mediated infection of T lymphocytes with the Human Immunodeficiency Virus-1 (HIV-1). Despite evidence that TCR and CXCR4 signaling pathways cross-regulate in important ways, CXCR4 signaling pathways in T lymphocytes are incompletely characterized. Moreover, to our knowledge, no detailed studies on the molecular mechanisms of chemokine or CXCR4 and TCR cross-regulation have been reported. Our recent results demonstrate that CXCR4 signaling regulates a close association between CXCR4 and TCR on the surface of T lymphocytes, and activates the ERK MAP kinase via a mechanism requiring the TCR and TCR associated signaling molecules. Our proposed experiments will therefore fill this gap in the current knowledge, by testing the central hypothesis that CXCR4 signals in T cells via a novel mechanism that involves CXCR4 associating with the TCR and using the TCR for signal transduction. Our specific aims are to (1) ask if the interaction between CXCR4 and the TCR is specific, (2) ask how CXCR4 signaling regulates the CXCR4/TCR complex and its movement into subcellular compartments where signaling takes place, and (3) ask if CXCR4 uses the TCR and other signaling proteins for ERK activation and costimulation of TCR signaling. The results of these studies will characterize novel mechanisms of CXCR4 signaling in T cells that are important for CXCA4-mediated gene expression, and that probably also contribute to CXCR4-mediated costimulation of T cell activation and HIV-1 pathobiology. In addition, whether or not the CXCR4/TCR complex is eventually found to have biological relevance, these experiments will address basic questions of how receptors oligomerize, and signal from within, subcellular compartments. Finally, these experiments will identify reagents that can be used to rigorously test the idea that CXCR4/TCR complexes regulate T lymphocyte biological functions. The results of these studies will therefore serve as a theoretical framework for better understanding and manipulating the interplay between T cell immune activation and T cell migration.